Gas turbine engine compressors usually comprise multiple stages of rotating blades that are used to compress air prior to combustion. Interspaced between the stages of rotating blades are vane stages which are used to direct compressor air flow in a manner that maximizes compressor efficiency. Some vane stages are adjustable so that the compressor can adapt itself to a variety of engine operating conditions by varying vane angle. Vanes in variable stages are therefore adjusted in order to direct compressor air flow; multiple vanes are ganged together for operation in unison by actuators.
In this type of mechanism, vane stems extend through the engine casing so that the vanes may be attached to vane arms. Vane stem bushings are used in order to provide an adequate bearing surface between vane stems and engine casing and to prevent engine compressor air from escaping the engine casing. Either plastic or metal bushings are used in this type of arrangement.
Several problems are related to use of conventional vane bushings. One serious problem is excessive bushing wear on the bushing outer diameter where it contacts the engine casing. This wear is a result of bushing auto-rotation due to engine vibration; rotational movement due to vibration can rapidly wear out the bushing and ruin its sealing and bearing properties. This type of vane bushing rotation occurs independently of vane stem rotation. Although the bushing may initially be tightly installed in the engine casing, slight movement during engine operation may produce wear that later allows substantial movement and bushing damage.
Another problem that has occurred with the user of conventional vane stem bushings is the loss of pressurized air from the compressor. This problem most often occurs at the inlet guide vane stage where there is insufficient internal air pressure to properly seat the bushing and form a good air seal. In higher compressor stages, where the internal air pressure is substantially higher, bushings will generally seat and form a tight air seal. Air loss from a multitude of bushings surrounding vane steams can have an adverse affect on compressor efficiency since it results in less air flowing through the engine to do useful work in the turbine.
Objects of this invention include providing a bushing assembly that restricts bushing rotation and reduces pressurized air leakage from the compressor.